Augmented reality systems offer a mechanism for providing a user with additional data about his or her environment. An example of such a system is a heads-up display (HUD) found in aircraft and automobiles. As is generally known, a HUD projects various navigational and/or operational data on a windshield or other transparent surface in a user's field of view. This allows a user to monitor various information without having to divert his or her gaze from the external environment.
Augmented reality systems have also been developed for use in a combat military environment. For example, commonly-owned U.S. patent application Ser. No. 11/000,934 (filed Dec. 2, 2004, titled “System and Method for Video Image Registration in a Heads Up Display,” published as Pub. No. US20060121993, and incorporated by reference herein) describes a system in which an image from a rifle-mounted video source is superimposed on an image seen through a pair of goggles. Sensors coupled to the rifle and to the goggles provide data indicating movement of the goggles and rifle. An image from the rifle-mounted source shows an external region within the source's field of view (FOV). The goggles have a wider FOV and provide an image that includes a portion showing the same region as is shown in the image from the rifle-mounted video source. The sensor data is then used to determine the relative orientation of the two sources and calculate a location for the rifle image within the image seen through the goggles.
Determining the relative orientations of two video sources based on inertial measurement unit (IMU) sensor data, as is described for at least some embodiments of the aforementioned application, can pose challenges. For example, many low-cost IMU sensors experience bias drift over time. Such drift can result in relative orientation errors of several degrees per hour. In order to prevent such errors from accumulating, the IMU sensors must be periodically recalibrated. This recalibration typically requires user action and can disrupt system operation. The ability to minimize the need for manually-initiated recalibration would be highly advantageous.
Additional advantages could also be obtained by increasing the types of information provided by a HUD within goggles worn by a user.